...
especially developer resources (www.apple.com/developer)
www.osxfaq.com
Just plain cool
Keychain Access (/Applications/Utilities).
Manages all your passwords. many apps talk to the keychain for you to get things like Exchange Server credentials, but it also lets you just put in passwords records yourself so you can keep records of frequent flyer numbers or bank account numbers.
Dashboard
Download oodles of widgets from Apple site.
Quicksilver (For Tiger OS)
...